1. Field of the Invention
The present invention relates generally to broadcast receivers, and more particularly to broadband tuners utilizing a local oscillator.
2. Related Art
Broadcast satellites transmit signals over multiple channels across a broadband spectrum. These satellites use standardized broadcast formats, such as Digital Satellite Service (DSS), used in North America, and Direct Video Broadcast (DVB), used in Europe. Both DSS and DVB broadcast over a broadband spectrum ranging from 12 to 14 GHz. When combined with receiving systems, these broadcast satellites create a direct broadcast satellite system (DBS).
A DSS/DVB receiving system generally includes an antenna, a low-noise block down-converter (LNB) and a down-conversion broadband receiver. Although there are many ways in which to implement the down-conversion broadband receiver, a direct down-conversion broadband receiver implementation is the most common in current generation tuners. The direct down-conversion broadband receiver comprises a direct down-conversion tuner and a baseband demodulator.
The LNB converts the 12–14 GHz band signal received by the antenna into a 950–2150 MHz band signal. This broadband signal is specified by the DSS/DVB standards. The direct down-conversion tuner directly converts one channel in this 950–2150 MHz band down to baseband, which consists of in-phase (I) and quadrature (Q) components. These I and Q components are then converted into digital data and demodulated by the baseband demodulator.
The direct down-conversion tuner in the direct down-conversion broadband receiver typically utilizes mixers to perform the down-conversion from 950–2150 MHz to I and Q components. These mixers require a local oscillator (LO), which typically comprises a frequency synthesizer with a step size equal to a receive-channel spacing. Such local oscillators are commonly implemented using a reference frequency generator, at least one voltage controlled oscillator (VCO), and a phase-locked loop (PLL) comprising a phase/frequency detector (PFD), a charge pump, a loop filter, and a divider. The reference frequency generator uses an external crystal.
Although the term “local oscillator” is frequently used to refer to only a VCO portion of a frequency synthesizer within a tuner, as used herein, the local oscillator refers to the frequency synthesizer within the tuner.
The signal from this local oscillator drives the mixers. By definition, the mixers in the direct down-conversion broadband receiver are driven by an input frequency that is equivalent to the carrier frequency of a selected channel. Thus the conversion to baseband is done in one step, thereby making it a direct down-conversion. Typically, the local oscillator operates at this same frequency input to the mixers.
However, some local oscillators are designed to operate at approximately twice the frequency of the carrier signal. The output of the local oscillator is then put through a 2-divider just prior to being fed to the mixers. This is done to avoid the many problems, such as cross modulation and direct current (DC) offset, which arise when the local oscillator operates at exactly the same frequency as the carrier.
In addition, when the signal is to be separated into I and Q components, using this divide by two approach offers another benefit. The 2-divider can be designed to easily generate the phase shift required to obtain the Q component, thereby avoiding the need for poly-phase filters or other complex designs. Thus with a 2-divider approach to the direct down-conversion tuner in a direct-down conversion receiver, both the I and Q components are obtained simultaneously, with a high degree of accuracy.
However, in DBS systems, the 2-divider approach is problematic. A DSS/DVB receiver must be able to receive channels over the entire broadband spectrum defined by the DSS/DVB standards. This broadband receiving spectrum causes the typical 2-divider approach to tuner design to create undesirable coupling between the local oscillator (LO) signal and the incoming radio frequency (RF) signal, within a DBS tuner chip. If the RF signal at twice a desired receive channel frequency is strong, this coupling can cause VCO pulling.
In addition, the RF signal at twice the desired receive channel frequency can mix with the LO signal and cause signal-to-noise-ratio degradation. For example, because the VCO typically generates a waveform, which is not a perfect sine wave, this generated waveform has additional harmonic content. These harmonic overtones can lead to composite triple beat. Thus, when some of the lower frequencies in the broadband spectrum of the DBS system are tuned to, the VCO wave form can interfere with higher RF channels, which in turn can created a product on the channel being tuned to, thereby reducing sensitivity of the tuner chip from the front end.
In addition to this VCO pulling problem with current broadband receivers, typical broadband receivers also consume an undesirable amount of power. With the DSS/DVB standards, the channel can be at any frequency in the specified signal range of 950 to 2150 MHz. A tuner chip designed to directly convert this broadband spectrum down to baseband typically relies on a reference frequency of 1 MHz or 500 KHz. The reference frequency used will commonly be set equal to a receive-channel spacing for the particular application. Generally, the reference frequency will not deviate significantly from these values. The reason for this is twofold. First, as the reference frequency is lowered, the phase noise performance required of the PLL increases, because the multiplication factor to the output becomes higher. Second, as the reference frequency is raised, the tuner chip loses granularity.
Because the tuning spectrum in DBS systems is so wide and any frequency in that range may be a channel, typical DBS tuner chips utilize a 16/17 prescaler in the divider in the PLL in order to handle the 1 MHz reference frequency case. This is done to avoid gaps in tuning range caused by a prescaler with too high of a modulus ratio. However, the use of a lower modulus prescaler in the tuner chip requires operation of counters in the divider at a higher frequency. Thus, this approach to local oscillator design creates a significant and undesirable power drain.
Therefore, what is needed is a system and method for local oscillator design for use in a broadband tuner, which avoids gaps in tuning range and VCO pulling, while also reducing the broadband tuner's power consumption.